System and method for fabricating Bragg gratings

ABSTRACT

A novel method and apparatus for fabrication of blazed and slanted fiber Bragg gratings is disclosed. The method comprises the step of simultaneously exposing the fiber with two mutually coherent light beams so as to create an interference pattern along a longitudinal axis of the fiber, wherein each one of said beams is brought into a line focus, which coincides with the core of the fiber. Further, the plane comprising the beams is rotated to provide a second angle relative to the fiber direction, said rotation giving rise to a blazing angle of the photo-induced grating elements.

FIELD OF THE INVENTION

The present invention relates to a method for photo-inducing a blazedgrating in an optical fiber comprising the step of simultaneouslyexposing the fiber with two mutually coherent light beams, whichintersects with an first angle in a plane comprising the beams andinterfere in a predetermined region of the fiber so as to create aninterference pattern along a longitudinal axis of the fiber. Theinvention further relates to an apparatus for performing the method.

BACKGROUND OF THE INVENTION

There is a rapidly growing demand for high-quality optical Bragggratings with arbitrary phase and index profiles, as these gratings arekey elements in many components that are used in WDM networks. Over thepast few years, several methods that improve the quality and theflexibility in the grating fabrication process have been developed. Astraightforward approach is to scan a UV beam over a long phase mask ina fixed relative position to the fiber. Non-uniform profiles can in thiscase be fabricated either by post processing the illuminated region orby using a phase mask that contains the appropriate structure. Complexgrating structures can also be synthesized by moving the fiber slightlyrelative to the phase mask during the scan.

In 1995, a novel versatile sequential technique for venting long andcomplex fiber gratings was demonstrated by R. Stubbe, B. Sahlgren, S.Sandgren and A. Asseh, in “Novel technique for writing longsuperstructured fiber Bragg gratings”, in Photosensitivity and QuadraticNonlinearity in Glass Waveguides (Fundamentals and Applications),Portland, PD1 (1995) and by A. Asseh, H. Storøy, B. E. Sahlgren, S.Sandgren and R. A. H. Stubbe, in “A writing technique for long fiberBragg gratings with complex reflectivity profiles”, J. Lightw. Techn.15, 1419–1423 (1997). The idea was to expose a large number of smallpartially overlapping subgratings—each containing a few hundred periodsor less—in sequence; where advanced properties such as chirp, phaseshifts and apodization were introduced by adjusting the phase offset andpitch of the subgratings. In the setup that was used in theabove-mentioned references, each subgrating was created by exposing thefiber with a short UV pulse while the fiber itself was translated at aconstant speed. The UV pulses were triggered by the position of thefiber relative the UV beams, which was measured by a standardhelium-neon laser interferometer.

Bragg gratings normally have their grating elements aligned normal tothe waveguide axis. However, there is an increasing interest inproducing gratings which have their elements at an angle to thewaveguide axis, known as blazed Bragg gratings. Such blazed Bragggratings are difficult to fabricate efficiently and with appropriateprecision with previously know methods and apparatuses.

For example, U.S. Pat. No. 5,730,888 and U.S. Pat. No. 5,042,897 bothrelates to methods and apparatuses to photo-induce blazed gratings inoptical fibers. The blazed gratings are formed by tilting the projectionsystem relative to the fiber. However, there are several problems withthese known methods. To be able to tilt the projection system relativeto the fiber the equipment becomes complex and costly. Further, it isdifficult to achieve an adequate focus on the fiber in the wholeexposure area. Hereby, the known methods becomes slow and inefficient,with a low through-put. Still further, with the known methods it is onlypossible to produce blazed gratings with a limited blazing angle,whereas blazed gratings with larger angles of inclination is verycomplicated to produce, or even not possible to produce at all.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand an apparatus for photo-inducing a blazed grating in an opticalfiber, which alleviates the above-mentioned problems of the prior art.

This object is achieved with an apparatus and a method as defined in theappended claims.

According to the invention there is provided a method for photo-inducinga blazed grating in an optical fiber comprising the step ofsimultaneously exposing the fiber with two mutually coherent lightbeams, which intersects with an first angle in a plane comprising thebeams and interfere in a predetermined region of the fiber so as tocreate an interference pattern along a longitudinal axis of the fiber.Further, each one of said beams is brought into a line focus, whichcoincides with the core of the fiber.

The invention presents a novel method for fabrication of advanced blazedfiber Bragg gratings. As opposed to prior art methods, the methodaccording to the invention does not tilt the projection system relativeto the fiber, but provides a blazed interference pattern in line focuswith the fiber.

Especially, the method could be used when said plane comprising thebeams, at least in the vicinity of the fiber, is rotated to provide asecond angle relative to the fiber direction, said rotation giving riseto a blazing angle of the photo-induced grating elements.

According to the inventive method, a blazing angle could be chosenarbitrarily, without any equipment restrictions. Further, the qualityand precision of the photo-induced pattern is improved, since the linefocus coincides with the fiber even at large blazing angles. Stillfurther, the control of the fabrication process becomes simplified,rendering the fabrication more efficient and less costly, and with ashortening of fabrication times for complex gratings.

Light is in the context of the application not limited to mean visiblelight, but a wide range of wavelengths from infrared (IR) to extreme UV.

Further, with optical fiber is in the meaning of this application to beunderstood any kind of optical waveguide made of a material which has arefractive index that can be permanently changed by exposure to light ofat least one predetermined wavelength.

By photo-induction is to be understood the process of exposing theoptical fiber of the above-mentioned type with light of thepredetermined wavelength so as to form a permanent refractive indexvariation in the fiber.

According to one embodiment of the invention, the beams are focused onthe fiber by means of at least one lens and the rotation of the beamplane is achieved by displacement of at least one of the beam incidencepositions on said lens. The beam incidence position could preferably bedisplaced in a direction essentially perpendicular to the fiberdirection. Hereby, the beams could be displaced on the lens, and stillbe focused on the fiber. Hence, the angle of incidence relative to thefiber could be varied without affecting the focus.

In an alternative embodiment, the beams are focused on the fiber bymeans of at least one curved mirror and rotation of the beam plane isachieved by displacement of at least one of the beam incidence positionson said mirror. The beam incidence position is preferably displaced in adirection essentially perpendicular to the fiber direction. Hereby, thebeams could be displaced on the mirror, and still be focused on thefiber. Hence, the angle of incidence relative to the fiber could bevaried without affecting the focus even in this embodiment.

The invention also relates to an apparatus for photo-inducing a blazedgrating in an optical fiber comprising a source for emitting light; abeam splitter for forming two mutually coherent light beams; a fiberholder for holding the fiber during exposure; and a projection systemfor making the beams intersect with a first angle in the exposure areaand thereby to interfere in a predetermined region of the fiber so as tocreate an interference pattern along the longitudinal axis of the fiber.The projection system further comprises means for focusing the beams sothat each one of said beams is brought into a line focus, whichcoincides with the core of the fiber.

With this apparatus, the method discussed above could be executed.Accordingly, a novel apparatus is presented for fabrication of advancedblazed fiber Bragg gratings. As opposed to prior art equipment, theapparatus according to the invention does not tilt the projection systemrelative to the fiber, but provides a blazed interference pattern inline focus with the fiber.

Especially, the means for focussing the beams could further comprisemeans for rotating the plane comprising the two light beams relative tothe fiber, at least in the vicinity of the fiber, to provide a secondangle relative to the fiber direction, said rotation giving rise to ablazing angle of the photo-induced grating elements.

According to the invention, a blazing angle could be chosen arbitrarily,without any equipment restrictions. Further, the quality and precisionof the photo-induced pattern is improved, since the line focus coincideswith the fiber even at large blazing angles. Still further, the controlof the fabrication process becomes simplified, rendering the fabricationmore efficient and less costly, and with a shortening of fabricationtimes for complex gratings.

In one embodiment, the means for focussing the beams comprises at leastone lens for focusing the beams on the fiber, the means for rotating thebeam plane comprising means for displacing the beam incidence positionon said lens for at least one of the beams. The beam incidence positionis preferably displaced in a direction essentially perpendicular to thefiber direction. Hereby, the beams could be displaced on the lens, andstill be focused on the fiber. Hence, the angle of incidence relative tothe fiber could be varied without affecting the focus.

The means for displacing the beam incidence position could comprise atleast one displaceable or rotatable reflecting mirror arranged in thebeam path for said beam, and preferably allowing parallax displacementof the beams.

In an alternative embodiment, the means for focusing the beams comprisesat least one curved mirror, and means for rotating the beam planecomprising means for displacing the beam incidence position on saidmirror for at least one of the beams. The beam incidence position ispreferably displaced in a direction essentially perpendicular to thefiber direction. Hereby, the beams could be displaced on the mirror, andstill be focused on the fiber. Hence, the angle of incidence relative tothe fiber could be varied without affecting the focus even in thisembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

For exemplifying purposes, the invention will be described in closerdetail in the following with reference to embodiments thereofillustrated in the attached drawings, wherein:

FIG. 1 is a schematic drawing of a fiber Bragg grating fabricationsystem according to an embodiment according to the invention.

FIG. 2 is a schematic drawing of a projection system in the exposurearea according to a first embodiment of the invention with the beams notbeing displaced, where FIG. 2 a is seen perpendicular to the fiber and 2b is seen in the longitudinal direction of the fiber.

FIG. 3 is a schematic drawing of the projection system in FIG. 2, butwhere the beams are displaced, where FIG. 3 a is seen perpendicular tothe fiber and 3 b is seen in the longitudinal direction of the fiber.

FIG. 4 is a schematic drawing of a projection system in the exposurearea according to a second embodiment of the invention, where FIG. 4 ais seen perpendicular to the fiber, 4 b is seen in the longitudinaldirection of the fiber, and 4 c is a side view from a directionperpendicular to the view in 4 a.

DESCRIPTION OF PREFERRED EMBODIMENTS

Further scope of the applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

The setup according to an embodiment of the invention is illustrated inFIG. 1. The base for the system according to the invention could e.g. bea system as described in “Fiber Bragg Gratings” by Raman Kashyap, p.55–101. Especially the system described with reference to FIG. 3.21serves as a good base. This description is hereby incorporated byreference.

However, other types of photo-inducing systems are conceivable for theinvention. A system according to the invention comprises in general alight source, a beam splitter of any kind, a projection system and afiber holder. The projection system could preferably comprise a mirrorarrangement, and sees to that the beams interfere with another in anexposure area so as to form a interference pattern in the fiber andhence corresponding grating elements. The light source could be acontinuous or pulsed wave laser. When a pulsed laser is used, the laseris preferably actuated when the distance moved by the fiber equals onegrating period. However, preferably a continuous laser is used, wherebythe interference pattern is caused to move in accordance with the fiberduring certain time periods. Between such time periods, the pattern israpidly reversed to the starting point so as to continue the gratingwriting. Hence, a generally un-interrupted writing process is achieved.Such movement of the interference pattern may be achieved by alterationof the phase difference between the beams, e.g. by displacing mirrors inthe beam path.

In a preferred embodiment of the invention, the fiber 1 to be exposed isplaced in a fiber holder 2 mounted on an airbearing born carriage 3,which is translated by a feedback-controlled linear drive. The positionof the translator stage relative the UV interference pattern is measuredwith a heterodyne interference detection system 4 utilizing a He-Nelaser as light source. The resulting spatial resolution is approximately0.6 nm, available over the translation length of about half a meter.

A light source 5, e.g. a frequency-doubled argon-ion laser emits light,such as 100-mW radiation of wavelength 244 nm, into a beam splitter,such as a half transparent mirror or prism, or a phase-mask, where it isdivided into two coherent beams. Thereafter, the beams are launched intoa projection system, such as a double Sagnac interferometer 6, whichgenerates the interference pattern forming the grating. In theprojection system, one or several cylindrical lenses focus the twointerfering beams into a line focus that coincides with the core of thefiber 1. Longitudinally, the focus could extend over about 100 microns,which roughly corresponds to 200 fringes for a Bragg wavelength of 1550nm resonance wavelength.

The apparatus is controlled by a control unit 8, which e.g. could beelectronically implemented in hardware. This control unit could in turnbe controlled by a software control unit 7, such as an conventionalpersonal computer.

Preferably, first displacing means 61 are provided, comprising e.g. astep motor, for controlling the angle between the interfering beams in aplane comprising the fiber. This could be used to change the period ofthe interference fringes to match the desired local pitch of thegrating. The angle change is preferably performed symmetrically for bothbeams so that the center fringe does not move its position. In apreferred setup, the resolution for this pitch variation is about 1.4 pmin resonant wavelength. Hereby, the chirp or distance between individualgrating elements could be accurately controlled.

In order to prevent unwanted exposure outside the actual grating, thesystem preferably also comprises a controllable shutter 9 in the UV beampath that is only open within the grating region during the writing.

Introduction of Controlled Blazing

According to the invention, the writing system further comprises means62 for focusing the beams so that each one of said beams is brought intoa line focus, which coincides with the core of the fiber. Said means forfocusing the beams further comprises means for rotating the planecomprising the two light beams relative to the fiber, at least in thevicinity of the fiber, to provide a second angle relative to the fiberdirection, said rotation giving rise to a blazing angle of thephoto-induced grating elements. However, the line focus is stillmaintained during said rotation.

The rotation could be provided by second displacing means 63 fordisplacing the beams, and preferably in a direction being obliquerelative to the fiber direction, and most preferably in a directionessentially perpendicular to the fiber direction.

In a first embodiment, as illustrated in FIGS. 1 and 2, the rotation isprovided by a lens 621, whereby the beam incidence position on said lensis displaceable for at least one of the beams. In FIG. 2, the beams arenot displaced, whereas they are displaced in FIG. 3. The displacementcould be provided by displaceable or rotatable mirrors 622 arranged inthe beam path. Hereby, the beams may be parallax displaced. However, aswill be appreciated by someone skilled in the art, other means fordisplacing the beams are conceivable as well.

The displacement of the beams does not influence the focusing of thebeams on the fiber. However, the beams being displaced provides an anglebetween the beams in a plane perpendicular to the fiber direction, as isillustrated in FIG. 3 b. Hence, the grating being photo-induced will beslanted, i.e. blazed, in a plane comprising the fiber but beingperpendicular to the plane illustrated in FIG. 3 a.

According to an alternative embodiment, as is illustrated in FIG. 4, therotation is provided by a curved mirror 623, whereby the beam incidenceposition on said mirror is displaceable for at least one of the beams.The displacement could be provided in the same way as is discussedabove. Accordingly, the grating being photo-induced will be slanted,i.e. blazed, in a plane comprising the fiber but being perpendicular tothe plane illustrated in FIG. 4 a, as is illustrated in FIG. 4 c.

The displacement is preferably performed on both beams, and mostpreferably in an essentially symmetrical fashion.

CONCLUSIONS

The invention provides a novel technique for fabrication of high qualityblazed fiber Bragg gratings. By the inventive method, a preciselycontrollable blazing may be introduced, without tilting the projectionsystem relative to the fiber or the like. As a result, the fabricationtime for all kinds of customized gratings is greatly reduced as comparedto earlier methods. The invention further allows for a very precisecontrol of the grating formation, and especially the introduction ofblazing.

The invention has now been described by way of embodiments. However,many alternatives are possible. For example, different types of beamsplitters are feasible, other types of means for translating the fibercould be used etc. Further, other means for displacing the beams arefeasible. Such alternatives are known from the prior art. It should beappreciated by someone skilled in the art that such alternatives arepart of the invention, such as it is defined by the appended claims.

1. A method for photo-inducing a blazed grating in an optical fiber,comprising: simultaneously exposing the fiber with two mutually coherentlight beams which intersect with a first angle in a plane comprising thebeams and which interfere in a predetermined region of the fiber so asto create an interference pattern along a longitudinal axis of thefiber, wherein each one of said beams is brought into a line focus,which coincides with the core of the fiber, and wherein said planecomprising the beams, at least in the vicinity of the fiber, is rotatedto provide a second angle relative to the fiber direction, said rotationgiving rise to a blazing angle of the photo-induced grating elements. 2.The method according to claim 1, wherein the beams are focused on thefiber by at least one cylindrical lens and the rotation of the beamplane is achieved by displacement of at least one of the beam incidencepositions on said lens.
 3. The method according to claim 2, wherein thebeam incidence position is displaced in a direction essentiallyperpendicular to the fiber direction.
 4. The method according to claim2, wherein the incidence positions of both beams are displaced.
 5. Themethod according to claim 4, wherein the incidence positions aredisplaced essentially symmetrically.
 6. The method according to claim 1,wherein the beams are focused on the fiber by at least one curved mirrorand rotation of the beam plane is achieved by displacement of at leastone of the beam incidence positions on said mirror.
 7. The methodaccording to claim 6, wherein the beam incidence position is displacedin a direction essentially perpendicular to the fiber direction.
 8. Themethod according to claim 6, wherein the incidence positions of bothbeams are displaced.
 9. The method according to claim 8, wherein theincidence positions are displaced essentially symmetrically.
 10. Themethod according to claim 1, wherein the fiber is translated through theexposure area where the beams intersect.
 11. An apparatus forphoto-inducing a blazed grating in an optical fiber, comprising: asource for emitting light; a beam splitter for forming two mutuallycoherent light beams; a fiber holder for holding the fiber duringexposure; and a projection system for making the beams intersect with afirst angle in the exposure area and thereby to interfere in apredetermined region of the fiber so as to create an interferencepattern along the longitudinal axis of the fiber, wherein the projectionsystem further comprises means for focusing the beams so that each oneof said beams is brought into a line focus, which coincides with thecore of the fiber, said means for focusing the beams further comprisingmeans for rotating the plane comprising the two light beams relative tothe fiber, at least in the vicinity of the fiber, to provide a secondangle relative to the fiber direction, said rotation giving rise to ablazing angle of the photo-induced grating elements.
 12. The apparatusaccording to claim 11, wherein the means for focusing the beamscomprises at least one lens for focusing the beams on the fiber, themeans for rotating the beam plane comprising means for displacing thebeam incidence position on said lens for at least one of the beams. 13.The apparatus according to claim 12, wherein the means for displacingthe beam incidence position comprises at least one reflecting mirror,which is at least one of displaceable and rotatable, arranged in thebeam path for said beam.
 14. The apparatus according to claim 12,wherein the means for displacing the beam incidence position on saidlens is adapted to displace the incidence positions of both beams. 15.The apparatus according to claim 12, wherein the means for displacingthe beam incidence comprises means for parallel displacement of thebeams.
 16. The apparatus according to claim 11, wherein the means forfocusing the beams comprises at least one curved mirror, and means forrotating the beam plane comprising means for displacing the beamincidence position on said mirror for at least one of the beams.
 17. Theapparatus according to claim 16, wherein the means for displacing thebeam incidence position is adapted to displace the incidence position ina direction essentially perpendicular to the fiber direction.
 18. Theapparatus according to claim 16, wherein the means for displacing theincidence positions is adapted to displace the incidence position ofboth beams.
 19. The apparatus according to claim 18, wherein the meansfor displacing the incidence positions displaces the incidence positionsof the beams essentially symmetrically.
 20. The apparatus according toclaim 11, further comprising: means for moving the fiber essentially inthe direction of a longitudinal axis of the fiber through the exposingarea where the beams intersect.
 21. The method according to claim 3,wherein the incidence positions of both beams are displaced.
 22. Themethod according to claim 7, wherein the incidence positions of bothbeams are displaced.
 23. The apparatus according to claim 13, whereinthe means for displacing the beam incidence position on the lens isadapted to displace the incidence positions of both beams.
 24. Theapparatus according to claim 13, wherein the means for displacing thebeam incidence includes means for parallel displacement of the beams.25. The apparatus according to claim 14, wherein the means fordisplacing the beam incidence includes means for parallel displacementof the beams.
 26. The apparatus according to claim 17, wherein the meansfor displacing the incidence positions is adapted to displace theincidence position of both beams.